Thunderbird refuses to show csv file--even with all Hidden Files/System Files set to show

I'm trying to move my address book from Outlook.com to Thunderbird. I created the file by using the Outlook.com export function, and I saved it in my Downloads folder. I opened Thunderbird, selected Tools>Import, clicked Address Book>Next. It lets me navigate to my Downloads folder, but the csv file is invisible. It shows up just fine when I'm not navigating via Thunderbird. I can see it in my Downloads folder. I went to my system's Show Hidden Files and made sure nothing was selected that would hide ANY files (system or otherwise). I rebooted the computer. I even moved the csv file to a temp folder on my desktop. Yet from within Thunderbird, when I navigate to that folder it is empty. I can't make Thunderbird let me see the file so I can choose it--even though I can see it just fine when I'm not in Thunderbird.